In an optical transmission circuit in an optical transmission system, optical interconnection, passive optical network (to be referred to as a PON (Passive Optical Network) hereinafter) system, or the like which can perform high-speed data transmission, an optical reception circuit which converts an optical signal into an electrical signal uses a transimpedance amplifier.
A transimpedance amplifier receives an input current Iin obtained by photoelectrically converting a received optical signal using a light-receiving element, converts the current into an output voltage Vout by using a transimpedance gain proportional to the value of a feedback resistor, and outputs the voltage.
In a transimpedance amplifier of this type, as the input current Iin increases, the amplitude of the output voltage Vout saturates, resulting in waveform distortion. In order to satisfy both requirements for high sensitivity and wide dynamic range characteristics, therefore, a conventional transimpedance amplifier decreases a transimpedance gain by reducing the value of a feedback resistor as the input current Iin increases, thereby obtaining the output voltage Vout with small distortion even at the time of reception of a large current.
A conventional transimpedance amplifier is disclosed in reference 1 (Saruwatari, Sugawara, and Ibe, “156 Mbps Burst Mode Optical Receiver”, IEICE transactions, 1997, B-10-128). As shown in FIG. 33, a transimpedance amplifier 300 is a circuit which includes an amplification circuit 311 and a gain switching circuit 312, and obtains an output voltage Vout by performing voltage conversion and signal amplification for an input current Iin output from a light-receiving element 100. The gain switching circuit 312 is configured such that a feedback resistor RF connects in parallel with a diode D1.
In the transimpedance amplifier 300, as the input current Iin increases, the voltage difference between the input terminal and output terminal of the amplification circuit 311 increases, and the diode D1 inserted in parallel with the feedback resistor RF is turned on. With this operation, since the value of the feedback resistor equivalently decreases, the transimpedance gain decreases. This makes it possible to avoid the saturation of the output voltage Vout even if a large current is input.
In addition, reference 2 (Japanese Patent Laid-Open No. 2000-252774) discloses another conventional transimpedance amplifier configured as a gain switching circuit which not only switches the values of one feedback resistor RF by turning on/off the diode but also switches/connects a plurality of feedback resistors. As shown in FIG. 34, a transimpedance amplifier 400 includes a transimpedance amplifier core circuit 410 and a gain switching determination circuit 420. The transimpedance amplifier core circuit 410 includes an amplification circuit 411 and a gain switching circuit 412, and performs voltage conversion and signal amplification for the input current Iin output from a light-receiving element 100. The gain switching determination circuit 420 controls gain switching by the gain switching circuit 412 in accordance with an output voltage Vout from the transimpedance amplifier core circuit 410.
More specifically, the gain switching circuit 412 comprises a plurality of feedback resistors whose switches connect in series. The gain switching determination circuit 420 obtains a gain switching signal SEL by monitoring the DC level of the output voltage Vout from the amplification circuit 411, and turns on/off the switch of the gain switching circuit 412 in accordance with the gain switching signal SEL, thereby switching the value of the feedback resistor.
In the case shown in FIG. 34, the gain switching determination circuit 420 comprises a high level holding circuit, low level holding circuit, and comparator 423. In this case, the high level holding circuit comprises an operational amplifier 421, capacitor C1, and diode D2, and holds the output voltage Vout from the transimpedance amplifier 400 at a high level. The low level holding circuit comprises an operational amplifier 422, capacitor C2, and diode D3, and holds the output voltage Vout at a low level. The comparator 423 performs switching determination by identifying that the potential difference between the high level and low level of the output voltage Vout which the two holding circuits hold becomes a predetermined value or more.